Effects of Ag Thickness and Deposition Temperature on Prevention of Cu Diffusion in Cu/Ag/Si System

Abstract

The introduction of copper (Cu) wiring or electrodes for LSI is advanced by using a buffer layer of a material such as SiO2, TiN or TiSiN to prevent the formation of copper silicide on a silicon (Si) substrate. When metal silver (Ag) serves as a buffer between Cu and a Si substrate, the Cu might remain stable on the Ag film. A fundamental study was performed on the necessary Ag thickness for preventing Cu diffusion into the Si substrate at room temperature and 300°C; the effect of lapse time after deposition was also studied. When Ag was deposited on the Si substrate with a thickness of less than 22 nm at room temperature, Ag islands with nearly round shapes were formed and many gaps were observed between them. Cu deposited on the Ag film reached the Si surface through these gaps and diffused into the Si substrate. When the Ag film was more than 30 nm thick, the Ag islands increased in size, the Ag film became thicker, and they came into contact with each other. The gaps between the Ag islands also narrowed, preventing the diffusion of Cu. The depth profile of the Cu deposited six months prior showed no change. When the deposition was carried out at a temperature of 300°C, Ag islands grew into tall three-dimensional (3D) islands, and the gaps between them broadened. Cu diffused deeply into the Si substrate.